The present invention relates to an analog scrambling/descrambling system.
Known analog scrambling/descrambling techniques are generally of two types, i.e., scrambling in the frequency domain and scrambling in the time domain. One of the earliest forms of frequency scrambling is simply interchanging the low and high frequencies of a speech signal that is band-limited to the range 300 to 3400 Hertz. A sophisticated form of frequency scrambling is the bandscrambler which divides the spectrum of a signal into a number of equal sub-bands and the signal is then scrambled by rearranging their order. Information identifying the number of sub-bands and the order of rearrangement is the key for descrambling the signal. Scramblers operating in the time domain are called time-element scramblers in which an analog signal is first divided into equal time periods called frames. Each frame is then sub-divided into small, equal time periods, or segments. The segments of each frame are then scrambled by rearranging their order.
U.S. Pat. No. 4,068,094 discloses a scrambling/descrambling technique that combines the frequency scrambling and time domain scrambling techniques.
However, in any of the prior art scrambling/descrambling techniques, there is a correlation between the amplitudes of the original speech signal and those of the scrambled signal. Because of this correlation, the rise and fall of the pitch of the transmitted spoken message are detectable by an interceptor. From the detected intonation, the interceptor can give a rough judgment of the contents of the message.